1. Technical Field
The invention concerns a temperature independent oxygen sensitive resistance material on a titanate basis.
2. Description of the Related Art
Ever stricter exhaust gas limit values, in combination with the pressure to reduce fuel consumption, force automotive manufacturers to develop new concepts for combustion engines. It becomes clear that the above-mentioned requirements are best brought together when operating with excess air (air ratio .lambda.&gt;1). Such modern "lean concepts" require exact information on the oxygen content found in the exhaust gas. The principle of the standard potentiometric .lambda. probe can be designed with difficulty and some great effort only for high oxygen concentrations as they occur in such lean exhaust gases.
In order to be able to measure the oxygen content of the exhaust gas even within the lean area, it was proposed, for example, in EP 0 191 627, in DE 38 41 611, as well as in [1] to set up amperometric probes in accordance with the current limit principle ("Limit current probe") from a material containing oxygen ions.
Such a limit current sensor is, for instance, also contained in Chemical Abstract 191612 for JP 02269948, such that a fixed electrolytic body fitted with two electrodes located opposite each other is introduced, where one of the electrodes consists of a Perovskite type oxide with the general formula L.sub.(1-x) A.sub.(x) Co.sub.(1-y) M.sub.(y) O.sub.(3-delta), with A being Sr, Ba, Ca and L being La, Ce, Pr, Nd, but where no generic type (Ti.sub.1-z Fe.sub.z) complex can arise, as for M only Fe, but not Ti and definitely no mixture of the two can be taken, which, however, according to the state of the art described below, would be fundamental for achieving a temperature independence from the working point of the oxygen partial pressure.
In addition, the operating mode of this sensor differs completely from resistive sensors and is based on an ion diffusion capability of an electrically insulating, but porous and for oxygen ions diffusion-capable layer, which is limited by a further gas-diffusion-controlling layer such that for an appropriate pump voltage a depletion in oxygen ions occurs, and the current flow to be measured now becomes dependent on the oxygen partial pressure. Any possibly occurring change in the ohmic resistance of a material of one of the electrodes will thus not be considered or utilized, and is rather regarded as an interfering factor.
However, according to DE 2334044 as well as pursuant to [2] there is also the option to utilize the oxygen partial pressure dependence of the electric conductivity of a metal oxidic material as a sensor effect, and to produce a sensor from this material, on the basis of whose electric resistance R it is possible to draw conclusions as to the oxygen partial pressure PO.sup.2 of the exhaust gas, and, on that basis again, as to the oxygen content in the exhaust gas.
Doped titanium oxide (TiO.sub.2) and strontium titanate (SrTiO.sub.3) were investigated with particular thoroughness (DE 37 23 051.
EP 0 365 567, [2]), as--due to their chemical stability--such titanium oxides are capable of withstanding the extreme operating conditions within the exhaust gas train of a combustion engine.
However, sensors made up of these compounds--as of most other metal oxides--feature a very significant temperature dependence of the electrical resistance, which requires an extensive heating control system, in combination with major construction measures that will reduce the influence and effects of sudden changes in temperature.
Therefore, it was considered in DE 42 02 146, DE 42 44 723, DE 43 25 183 and EP 0 553 452 to use doped or undoped cuprates in future such as e.g. La.sub.2 CuO.sub.4+y, as their electrical conductivity--specifically in the area of high oxygen content values, that is for .mu.&gt;1,--is temperature independent. For applications within the exhaust gas train, however, cuprates are not suitable, as they are chemically not all that stable and decompose at high temperatures and/or low oxygen partial pressures, such as e.g. during short term operation with a "rich" mixture (.lambda.&lt;1).
Lanthanferrites doped with alkaline earth, known from DE 44 18 054, feature a significantly higher chemical stability than cuprates. Their electrical conductivity also features a lower temperature dependence within the lean exhaust gas range (.lambda.&gt;1) when compared to SrTiO.sub.3. However, sensors made from these materials show a greater temperature dependence of the electrical resistance value than sensors on a cuprate basis.
In EP 0 062 994 or in U.S. Pat. No. 4,454,494 with the same priority, Williams et al. proposed to replace the titanium (Ti) in SrTiO.sub.3 in part by iron (Fe), and found out that sensors made from the compound SrTi.sub.0.7 Fe.sub.0.3 O.sub.3-.delta. in lean atmospheres above 500.degree. C.-600.degree. C. have almost no temperature dependence of the electrical resistance but show a dependency on partial oxygen pressure according to R.about.pO.sub.2.sup.-1/5. Own measurements confirmed this but also showed that the temperature independence of the electric resistance will exist only at a partial oxygen pressure around 10.sup.-2 bar (.lambda..apprxeq.1.055).